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Fault line pressure now closer to Tokyo

By Robin McDowell
Associated Press / March 22, 2011

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JAKARTA, Indonesia — The recent monster quake that hit northeastern Japan altered the earth’s surface, geologists say, loading stress onto a different segment of the fault line much closer to Tokyo.

Scientists are quick to point out that this does not mean a powerful earthquake is necessarily about to strike the Japanese capital. Even if it did, the structure of the tectonic plates and fault lines around the city makes it unlikely that Tokyo would be hit by a quake anywhere near the intensity of the 9.0-magnitude temblor that struck March 11, said Roger Musson of the British Geological Survey.

But, given the vast population — Tokyo and its surroundings are home to 39 million people — any strong quake could be devastating.

“Even if you’ve got, let’s say, a 7.5, that would be serious,’’ the seismologist said.

Japan is located on the Ring of Fire, an arc of volcanoes and fault lines spanning the Pacific Basin, and is regularly hit by earthquakes.

But before last week’s quake, the largest to hit the country since it started keeping records 130 years ago, few geologists considered Japan to be a strong candidate for a 9-plus earthquake, said Andrew Moore of Earlham College in Richmond, Ind.

There is mounting evidence, however, that Japan has been struck by several severe quakes in the last 3,500 years, most in the northern reaches of the country. Sand deposits indicate that several quakes have spawned 30-foot-high waves that slammed into the northern island of Hokkaido, he said, the most recent in the 17th century.

Similar deposits underlie the city of Sendai, the area rocked last week. The most recent is from an 869 AD tsunami that killed 1,000 people and washed more than 2 1/2 miles inland.

Even weaker quakes that hit Tokyo have caused significant damage.

But last week’s tremor changed the coastal landscape, and not just above sea level. It created a trench in the sea floor 240 miles long and 120 miles wide, as one tectonic plate dove 30 feet beneath another, said Eric Fielding of NASA’s Jet Propulsion Laboratory. While that relieved stress at the breaking point, it appears to have piled pressure onto adjacent segments, said Brian Atwater, a geologist with the US Geological Survey. The added strain could now trigger a strong, deadly aftershock on Tokyo’s doorstep.

The shift in pressure is a common occurrence after strong quakes and happened after the 2004 mega-earthquake and tsunami off Indonesia that killed 230,000 people in a dozen nations. Three months later, an 8.6-magnitude quake erupted farther down the fault line, killing 1,000 people on sparsely populated Nias island.

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